In controlled-potential coulometry and related techniques, the empirically evaluated constant of proportionality defined by the equation $$s_{\text{B}}=-\frac{1}{c_{\text{B}}}\frac{\mathrm{d}{c}_{\text{B}}}{\mathrm{d}t}$$ where $c_{\text{B}}$ is the bulk concentration of the substance B, and $\frac{\text{d}{c}_{\text{B}}}{\text{d}t}$ is the rate of change of that concentration, resulting from the consumption of B by reduction or oxidation at the working electrode.